Thus, in an elliptical apartment, if the sounding body be placed in one focus, the sound will be heard much louder by a person situated in the other focus of the ellipsis, that in any other part of the room. In this case, the effect is so powerfifl, that even when the middle part of the room is wanting, the sound expressed in one focus will be heard by a person situated in the other, but hardly at all by those who stand in the intermediate space.
Without attempting to explain the manner in which the vibrating air impinges upon, and is sent back by, the reflect ing body, which would lead us too far into the science of acoustics, we shall briefly notice the following ascertained facts.
If a person standing before a high wall, a bank, a rock, &c., at a certain distance, and uttering a word with a pretty strong voice, or producing by a hammer, stone, &c., any short, sharp sound, hear a repetition of that word or sound, he will find that the time elapsed between his uttering the word, or striking with the hammer, and hearing of the echo, is equal to the time that a sound is known to employ in going through an extension of twice the distance between him and the reflecting wall, rock, &c.; for the vibratory motion of the air must proceed from the sounding person to the wall, &c., and back again from the latter to the former. Now, sound is known to travel at the rate of about 1,125 feet in a second of time ; therefore, if the person who expresses the word, or any sound whatever, stand at the distance of 1,125 feet from the echoing wall, &c., two seconds of time must elapse between his uttering the sound and his hearing the echo. If the distance be equal to 4,500 feet, then eight seconds of time must elapse between the uttering of the sound, and the arrival of the echo; and so on. But the same original sound and the echo may be heard by persons at different distances, both from the original sounding-place, and from the reflecting body. The effect, however, will not be exactly uniform, fbr those who are nearer to the reflecting body, will hear the echo sooner than persons more remote. A situation may be easily found, from which they will hear both the original sound and the echo at the same instant, and as both sounds coalesce, they will only appear as one loud sound, without the echo.
But though several persons, in different situations, may hear the echo of the same sound, yet the echo will be heard better in one particular direction than in any other ; now if two straight lines be drawn from the middle of a reflecting surface, one to the place from which the original sound pro ceeds, and the other to the above-mentioned best direction, those two lines will be found to make equal angles with the surface. Hence it appears, that in the reflection of sound,
the angle of incidence is equal to the angle of reflection. Therefore, if a person wishes to hear the echo of his ow n voice in the best possible manner, he must stand in a direc tion perpendicular to the reflecting surface. And this shows, that though sound proceeds from an original sounding body, or from a reflecting surface, in every direction ; yet a greater quantity of it proceeds in some particular direction than in any other, which is probably owing to the original impulse being given to the air more forcibly in one direction than in another, or from want of perfect freedom in the aerial fluid.
Several phenomena may be easily explained upon the above-mentioned property of sound : for instance, several reflecting surfaces are freqtiently so situated with respect to distance and direction, that a sound proceeding from a certain point is reflected by one surface first, then by a second, soon after by a third, and so on, but by all in one direction; in which case a multiplied tautological echo is produced ; that is to say, the same word is heard repeated several times succes sively in the same tone and accent ; the expression of one will appear like a peal of laughter; a musical instru ment, properly played, will produce an agreeable repeti tion of as many instruments of the same sort, imitating each other.
According to the various distances of the speaker, a reflect ing object will return the echo of several, or of a few sylla bles, for all the syllablesmust be uttered before the echo of the first syllable reaches the ear; otherwise it will make a confusion. The farther the reflecting object is, the greater Dumber of syllables will the echo repeat ; but the sound will be enfeebled nearly in the same proportion, till at last the syllables cannot be heard distinctly. When the reflecting object is too near, the repetition of the sound arrives at the ear whilst the perception of the original sound still continues, in which case, an indistinct sounding noise is heard. This effect may especially be observed in empty rooms, passages, &e., because, in such places, several reflections from the wall to the hearer, as also from one wall to the other, and then to the hearer, clash with each other, and increase the indistinct ness.